Rick Santorum’s withdrawal from the Republican presidential race Tuesday once again drew attention to the rare genetic condition of his 3-year-old daughter, Bella.

As the former U.S. senator from Pennsylvania has said repeatedly, his daughter has already beaten the survival odds for children with trisomy 18. Close to 60 percent of newborns with that condition die in the hospital within a few days of birth. Only 5 to 10 percent survive a year. Only 1 percent live to their 10th birthdays, with survival that long much more likely in girls.

Bella, whose full name is Isabella, was born in 2008. The family has not provided details of her condition, although she was recently hospitalized, reportedly for pneumonia, for the second time this year.

Children with trisomy 18 (also sometimes called Edwards syndrome) have three copies of chromosome 18 instead of two. Human beings have 22 pairs of regular chromosomes (called “autosomes”) and one pair of sex chromosomes, with one-half of each pair contributed by the father and the other half by the mother. The cause of the condition is unknown, although, as with similar defects, children with older mothers are at higher risk.

Trisomy 18 is one of the three most common chromosomal abnormalities, although it is quite rare, occurring in roughly 1 out of every 8,000 live births.

It can be discovered before birth by microscopic examination of amniotic fluid and by ultrasound. Three-quarters of fetuses diagnosed are miscarried or stillborn between 12 weeks of gestation and birth. Some are also electively aborted.

Nearly all babies with trisomy 18 have malformation of several organs. Almost all have abnormal hearts, with a hole between the heart’s largest chambers, the ventricles, being the most common defect. One-third have a single horseshoe-shaped kidney and one-fifth have an abnormal esophagus. Mental retardation is common but not universal, and many have distinctive facial features such as low-set ears.

About 90 percent have low birth weight and some are premature, which further threatens their health in the days after birth.

Infections are a common and recurring problem in babies with trisomy 18 who survive.

Last year, researchers at Tufts University analyzed what genes were either more or less active in fetuses with trisomy 18 than in those without the condition. They identified 251 whose “activation” was significantly different.

Curiously, only 3 percent of those genes were on chromosome 18. That suggests many problems are the result of improper signals sent by chromosome 18 genes to presumably normal genes on other chromosomes.

Among the affected genes are those that regulate the immune system, which probably explains the unusual susceptibility of these children to infection.

Staff researcher Lucy Shackelford contributed to this report.